This invention describes a process for the removal or reduction in concentration of higher hydrocarbon impurities from methane and/or ethane. These impurities are typically found in methane that is derived from natural gas but may also be found in other gas mixtures. The higher hydrocarbon impurities are normally saturated aliphatics with from two to four carbon atoms. Thus, ethane, propane, butane, and isobutane are the predominant organic impurities in natural gas-derived methane. Higher hydrocarbons other than C.sub.4 may also be present.
The C.sub.2+ impurities are present in natural gas when it is removed from the ground together with inorganic contaminants such as nitrogen, hydrogen sulfide, carbon dioxide, and water. The inorganic components can be removed or reduced in concentration by a wide variety of chemical or physical procedures. The organic impurities can be and normally are separated from the natural gas for economic reasons. The C.sub.2+ hydrocarbons have a greater economic value than does methane itself.
The heavier hydrocarbon components may be removed from the methane and/or ethane by either physical or chemical procedures. Physical procedures for hydrocarbon removal are represented by distillation and absorption. Both methods are based on equilibrium processes which require multiple steps for each additional increment of hydrocarbon removal. In addition, the physical methods generally are operated under cryogenic conditions which are comparatively costly. As a consequence, physical methods of separation are terminated at an economic barrier depending on the economic advantage of removing an additional increment of higher hydrocarbons or at the desired purity level which may be beyond the economic barrier. It is particularly costly to purify methane and/or ethane beyond the economic barrier.
An alternative to physical methods of hydrocarbon removal from methane is the use of catalytic reduction. In this process, excess hydrogen is mixed with the impure gas stream which is then fed to a catalytic reactor in which the higher hydrocarbons are converted to methane. This procedure does not extract the value of the hydrocarbons but does reduce their concentration. However, hydrogen is a relatively expensive material and is not always conveniently available.
In many cases the higher hydrocarbon content of methane and/or ethane does not affect its use and in some cases it may even be beneficial. However, in certain cases, such as the use of methane ballast in an ethylene oxide reactor, the ethane concentration must be controlled below certain levels and the complete absence of propane and higher hydrocarbons is preferred. To accomplish this control of the higher hydrocarbon concentration via the extant physical or chemical procedures would be complicated and expensive or both. Thus it can be seen that there is a need for a method which is capable of treating gas streams of methane and/or ethane containing relatively small amounts of higher hydrocarbons to reduce the concentration of higher hydrocarbons in methane and/or ethane to very low levels without the necessity of using the relatively expensive and sometimes unavailable hydrogen which is required in the commercially used catalytic processes.